HO trucks (scale 1 : 87) seem to be much too small for Gn15 models ( approx. 1 : 24 scale) In most cases they look out of scale or simply sily, if used.
After all there are a few makers offering scale Gn15 models. Here is a Schomberg (made in Canada) flatcar with a HO bogie load.
Unfortunately the Schomberg moulds seem to be quite worn out meanwhile. So be prepared, to replace some nuts and bolts with suitable castings from makers like Ozark, Cambrian, Gandt Line etc.
Barrel transports seem to be a common sight on Gn15 layouts.
My examples were made from Ralph Brammer (Rockleybottom) parts a few years ago.